Subscriber line digital multiplexers are well-known; for example, there is the SLM (Subscriber Line Multiplexer) system being manufactured and sold by the Bell System in the U.S. and described generally in the Bell Laboratories Record for March, 1972 on pages 80-86 in an article by I.M. McNair. This system includes combined time division transmission and space division crossbar switching at the central office terminal. Adaptive delta modulation is used for digital data transmission across a span to a remote terminal for demodulation and passage over a wire path to the subscriber. Space division concentration from a maximum of eighty lines is provided for the 24 channels or trunks using crossbar switches. A number of similar systems are on sale in the U.S. at the present time, these primarily being PCM systems using twenty-four channels with a limited number of remote terminals available.